1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer.
2. Description of the Related Art
In recent years, there has been a demand for further improving the quality of images output from color image forming apparatuses such as color copying machines. Gradation of density and the stability of density gradation in particular largely influence a person in determining the quality of an image.
Images obtained by a color image forming apparatus such as mentioned above vary in density if the environment changes or if parts of the apparatus are changed during long-time use. In particular, in the case of an electrophotographic color image forming apparatus, there is a possibility of occurrence of variation in density even with a slight environmental change and hence a possibility of color balance being lost. It is therefore necessary for the apparatus to have means for maintaining constant densities at all times.
In conventional color image forming apparatuses, therefore, measures described below have been taken. A toner image (toner patch image) for density sensing is formed by a toner in each color on an intermediate transfer member, a photosensitive member or the like, and the density of the unfixed toner patch image is sensed with an unfixed toner density sensor. Density control is performed by feeding back the result of sensing to process conditions including the amount of exposure and the development bias. Images are obtained with stability in this way.
In the above-described density control using the density sensor, a toner patch is formed on an intermediate transfer member, a drum or the like and is sensed, and no control is performed with respect to a change in color balance of the image due to transfer onto and fixation on a transfer medium performed thereafter. Therefore, the above-described density control using the unfixed toner density sensor cannot be suitably performed with respect to a change in an image transferred onto a transfer medium when the image is fixed.
A color image forming apparatus equipped with a density sensor (color sensor) for sensing the density or color of a toner patch after fixation on a transfer medium (e.g., one disclosed in Japanese Patent Laid-Open No. 9-305058) has therefore been proposed.
However, a problem described below may arise in forming an image at a high speed in a color image forming apparatus equipped with a density sensor (color sensor) for sensing the density or color of a toner patch after fixation on a transfer medium.
That is, there is an upper limit to the electric power provided in the image forming apparatus as a source of an amount of heat usable for fixation. Therefore the temperature of a fixing unit decreases during a high speed continuation of color output. In such an event, image output can be performed if the reduction in the fixing temperature of the fixing unit (from a temperature adjustment point T3 to another temperature adjustment point T0) is within an allowable temperature range such that a fixing effect high enough to prevent separation of toner can be ensured. However, even if the temperature of the fixing unit is within the allowable temperature range, a slight variation in the degree of melting of toner due to a change in fixing temperature may occur and appear as a change in tint of the resulting output image. Consequently, even at a fixing temperature within the allowable temperature range, there is a possibility of a considerable reduction in image quality due to a change in the fixing temperature.